1. Field of the Invention
The subject invention relates to optics and, more particularly, to the display of images from a continuously moving image carrier.
2. Description of the Prior Art
In 1895 the pioneering brothers Lumiere made their first demonstration of motion picture projecton on a screen. Shortly thereafter they devised an improvement of their cinematograph for the projection of motion pictures from a continously moving film. As may, for instance, be seen from their U.S. Pat. No. 634,560, issued Oct. 10, 1899, this machine already manifested in some mechanistic form fundamental concepts that would benefit a modern motion picture projector.
For one thing, the film motion was continuous, which imposes much less wear and tear on film and film drive mechanism than an intermittent film advance. Also, continuous film motion would have been more compatible with the provision and playback of sound recordings on the film and, half a century later, with the scanning of motion pictures by television equipment.
Secondly, the continuous Lumiere cinematograph operated with a single motion compensating element. This compares very favorably in terms of equipment, maintenance, size and cost to the many proposals that employ a multitude of rotating lenses, mirrors or prisms and that are thus very expensive, bulky and in continual need of repair or maintenance by skilled and specially trained personnel.
Two further elements of the Lumiere apparatus, namely a sawtooth cam for periodically advancing and resetting the film motion compensating element, and a spring for biasing that element, set the path for further development up to the present time. As will presently be shown, the chronic inability of the art to break away from Lumieres' cam and spring concept bears the responsibility for the persistent predominance of the intermittent projector over the potentially much more advantageous continuous machine.
A sawtooth cam became a standard device for driving single-element compensators, as is particularly well illustrated in the first figure of U.S. Pat. 2,718,549, by C. F. Mattke, issued Sept. 20, 1955. After many years of development, it was recognized that a feature for correcting effects of film shrinkage and other imperfections is an indispensable necessity. Accordingly, an error correcting element was added to the film motion compensating element, as may be seen from U.S. Pat. 2,227,054, by A. V. Bedford, issued Dec. 31, 1940. Since purely mechanical systems proved incapable of satisfactory operation, proposals were made to provide the error-correction feature by an electronic servo system.
According to one of these proposals (see U.S. Pat. 2,666,356, by R. E. Graham and C. F. Mattke, issued Jan. 19, 1954), Bedford's film motion compensating element was replaced by a rotating drum structure which had a crown of spring-biased rocking mirrors for routinely compensating for the continuous film motion irrespective of film shrinkage and other imperfections occurring during the display. An error correcting element in the form of an auxiliary mirror was provided in addition to the mirror drum. This auxiliary mirror was actuated by an electronic servo system which sensed errors in the stationary positioning of the projected image in at least two directons and which adjusted the auxiliary mirror accordingly.
Shortly after this proposal the further suggestion was made to combine both the film motion compensation function and the error correction feature in one and the same mirror element (see U.S. Pat. 2,770,163, by C. F. Mattke, issued Nov. 13, 1956). To this end, the mirror element was routinely driven by a cam to perform the requisite continuous motion compensation, and a servo system of the type mentioned in the preceding paragraph was made to act on that same mirror element for the purpose of error correction.
In the meantime the search for a solution which would overcome the inadequacies of mechanical drive systems for the compensator yielded the replacement of the erstwhile sawtooth cam by an electronic sawtooth generator, as may be seen from U.S. Pat. 2,506,198, by D. Charles, issued May 2, 1950. In the case of electromechanical compensator elements, it proved, however, difficult to effect an error correction by changing the routine sawtooth oscillation of the compensator.
In time it was found that the presence of a sawtooth function generator was not indispensable. With a spring-biased compensator element, an electric current of gradually increasing magnitude could automatically be provided by a servo amplifier that was connected to a photocell sensor which detected movements of an illuminated sprocket hole (see U.S. Pat. 2,843,006, by A. W. Tyler, issued July 15, 1958; and U.S. Pat. 3,067,284, by J. L. E. Baldwin, issued Dec. 4, 1962).
It had become clear by this time that a spring bias of the compensator element could not practically serve as the only agency for resetting this element. Rather, a recurring resetting current came to be applied to the compensator element in order to assist the bias spring in resetting the compensator, and also in order to permit a synchronization facility in the case of motion picture scanning for television purposes (see the above mentioned Tyler Pat. 2,843,006).
The application of reset pulses permitted the use of weaker bias springs. This yielded some reduction in the amplitude of the driving sawtooth current, since the drive of the compensating element has to work against the bias spring stiffness. A further improvement resulted from the realization that the compensator element could be biased in a mid-position between its outer limits of reciprocating travel. In this manner, the extreme amplitude of the drive current was virtually cut in half. However, the resulting improvement was largely nullified by the lack of a restoring force against overshooting of the compensator advance during the initial one-half of the compensator advance.
Accordingly, all these proposals, representing an immense development effort for the better part of the present century, did not yield the solution required for a displacement of the intermittent motion picture projector on a broad scale. As initially indicated, the responsibility for this failure rests with the persistent inability of the art to break away from Lumieres' cam and spring concept when moving from the mechanical age into the electronic era.
The belief in the indispensable necessity of a bias spring has become that engrained that many illustrations of prior-art continuous film motion projectors do no longer show the spring; assuming tacitly its presence. Even systems that were constructed apparently with a view to bidirectional image correction still fell back to the traditional cam (or sawtooth current) and spring concept for the mirror drive, thereby reopening the door to the host of obstacles besetting that approach.
The advent of television and the use of video equipment sawtooth generators for compensator deflection purposes contributed further to the adherence to sawtooth drive concepts, as may be seen from the LASER DISPLAY STUDY by C. E. Baker and H. W. Parker, Technical Report No. RADC-TR-65-169 (Rome Air Development Center, Griffiss Air Force Base, New York, July 1965).
In consequence, workers in the field wishing to elaborate on more promising servo loop principles were faced with an alternative between proposals which required sawtooth driving currents of such high amplitudes that overheating of the drive coil or thermal warping of the mirror resulted, and suggestions which left significant portions of the mirror advance practically uncorrected.